1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as copiers, printers, facsimiles and combined machines thereof.
2. Description of the Related Art
It is a common practice in the electrophotographic image forming apparatus such as copiers, printers, facsimiles and combined machines thereof that the developing device supplies the toner to an electrostatic latent image formed on the image carrier so as to form thereon a toner image corresponding to the electrostatic latent image.
After the toner image thus formed on the image carrier is transferred by the transfer device to the transfer receiving material such as a recording medium or an intermediate transfer belt, the toner recovery device recovers from the surface of the image carrier the toner remaining thereon after the transfer of the toner image. There is also known an apparatus which employs the toner return device for returning to the developing device the toner recovered by the toner recovery device so that the recovered toner is put to effective use.
The above developing device includes a developing device of a single component development system using a toner alone as the developer and a developing device of a two-component development system using a developer containing the toner and the carrier. The developing device of the two-component development system using the developer containing the toner and the carrier is superior in terms of quick and adequate toner charging for facilitating high-speed image formation.
In the above-described developing device of the two-component development system, the developer containing the toner and the carrier is mixingly agitated to charge the toner by contact with the carrier. The developer containing the charged toner is retained on the developer carrier having the magnet member equipped with the magnetic poles mounted on the inner periphery thereof. The developer carrier is rotated to deliver the retained developer to the image carrier.
The toner of the developer retained on the developer carrier is supplied to the image carrier by applying the developing bias voltage from the bias voltage source between the developer carrier and the image carrier. Thus, the toner image corresponding to the electrostatic latent image is formed on the surface of the image carrier.
In a case where the developer is decreased in toner content in consequence of the above-described toner supply to the image carrier, a toner replenisher replenishes the developing device with the toner.
The above developing device of the two-component development system has the following problem. As the developer is used over a long period of time, the carrier in the developer is gradually deteriorated so as to become incapable of adequately charging the toner. As a result, the developing device becomes incapable of accomplishing proper image formation.
It has been a conventional practice to remove the developer suffering from the carrier deterioration from the developing device for replacement with a fresh developer or to replace the developing device per se with a new one.
However, in the case where the developer suffering from the carrier deterioration is removed from the developing device for replacement with a fresh one, or where the developing device per se is replaced with a new one, the developer actually contains a substantial amount of toner that is still usable. Namely, there is a problem that the usable toner remaining in the developer is wastefully discarded.
More recently, therefore, there has been proposed an apparatus wherein the above developing device is provided with separation means for separating the toner from the developer to be removed, as disclosed in Japanese Patent Publication No.3581720. The separation means includes a developer collection portion and a toner recovery portion. A charge of the opposite polarity to that of the toner is applied to a developer collection sleeve so as to separate the toner from the developer to be removed. The separated toner is returned to the developing device for recycling.
However, in the case where the developing device is provided with the developer collection portion and the toner recovery portion as the separation means for separating the toner from the developer to be removed, and the charge of the opposite polarity to that of the toner is applied to the developer collection sleeve for separating the toner from the developer to be removed, the developing device need be equipped with many devices. Thus, the developing device is increased in cost and in size.